1. Field of the Invention
The present invention relates to a process for the manufacture of seamless pressure vessels from a CrMo steel.
2 Description of the Prior Art
The manufacture of such pressure vessels has been part of the prior art for many years. The limit conditions to be observed in the fabrication of such vessels are contained in published technical standards and documentation, e.g., in VdTUV Material Specification 431 (Edition 03.88). Apart from the composition of the material to be used, the above-referenced Material Specification prescribes, among other things, a hardening treatment and a tempering of the finished pressure vessel. The hardening, in the context of a quenching and tempering, occurs after an annealing at 830.degree. to 880.degree. C. and a hold time of at least one minute per mm of wall thickness of the pressure vessel, but with the hold time being at least 15 minutes, with a subsequent quenching in oil, which has a maximum temperature of 50.degree. C. The tempering is performed at 530.degree. to 680.degree. C. with a hold time of at least 2 minutes per mm of wall thickness, but with the hold time being at least 30 minutes, and with a subsequent cooling in air. The pressure vessels must exhibit at least the following essential material characteristics:
______________________________________ 0.2% proof stress R.sub.p0.2 &gt;755 N/mm.sup.2 Tensile strength R.sub.m = 880-1030 N/mm.sup.2 Elongation after A.sub.2" &gt;14% fracture Notch impact .sup.a K-20.degree. C. transverse &gt;25 J/cm.sup.2 toughness ______________________________________
The characteristics of a particular alloy are customarily determined as an average of the measurements performed on three test pieces. The notch impact toughness may be determined using the well know Charpy V-Notch (or "CVN") Impact Test. For a pressure vessel which is 229 mm in diameter and which has a minimum wall thickness of 5.8 mm, manufactured from a customary material, i.e., 34 CrMo4, having the following analysis:
0.36% C PA0 0.21% Si PA0 0.68% Mn PA0 0.014% P PA0 0.007% S PA0 1.03% Cr PA0 0.24% Mo PA0 0.0081% N PA0 0.05-0.30% C PA0 0.01-0.40% Si PA0 0.80-1.50% Mn PA0 0.01-5.00% Cr PA0 0.01-2.0% Ni PA0 0.01-1.0% Cu PA0 0.01-2.0% Mo PA0 up to 0.10% Al PA0 up to 0.50% V PA0 up to 0.50% Ti PA0 up to 0.50% Zr PA0 up to 0.50% Nb PA0 0.0003-0.0050% B
which was quenched in oil after a 15 minute annealing at 850.degree. C., which was then tempered for 30 minutes at 630.degree. C., and which was finally cooled in air, the following material characteristics were measured: EQU R.sub.p 0.2=812 N/mm.sup.2 EQU Rm=938 N/mm.sup.2 EQU A.sub.2" =15.6% EQU aK-20.degree. transverse=87 J/cm.sup.2 EQU aK-40.degree. transverse=52 J/cm.sup.2 (with brittle fractures)
The values for the 0.2% proof stress were scattered, the scatter range being 85 N/mm.sup.2. The same is true for the hardness values, which were scattered over a range of 40 HB. As used herein, the terms "scatter", "scatter range" and "scatter band" (in the German language, "die Briete des Streubandes") indicate that measured values are spread over some range of deviation from an average value.
For special applications, it may be desirable to significantly increase the strength characteristics of certain particular pressure vessels, while effecting a simultaneous retention of the toughness characteristics, or to significantly increase the toughness characteristics, while effecting a simultaneous retention of the strength characteristics. The present invention is particularly well suited for such applications.
In accordance with a known manufacturing process, an improvement in the toughness can be achieved with an appropriate variation of the heat treatment, but the improvement in toughness is normally always accompanied by a significant reduction in strength, and vice-versa.